The present invention relates to electronic circuits that utilize shunt regulators. In particular, the present invention relates to a method and apparatus that provides for enhanced protection from electrostatic discharge (ESD) in shunt regulators.
Static electricity has been an industrial problem for centuries including such examples as paper and grain mills. The age of electronics brought with it new problems associated with static electricity and electrostatic discharge. Additionally, as electronic devices have become faster and smaller, their sensitivity to electrostatic discharge (ESD) has increased. Today, ESD impacts productivity and product reliability in virtually every aspect of the electronic environment. Despite a great deal of effort during the past decade, ESD still affects production yields, manufacturing costs, product quality, product reliability, and profitability. The costs of damaged devices can range from only a few cents for a simple diode device to several hundred dollars for complex hybrid microelectronic circuits.
An example of an ESD test circuit (100) for an electronic circuit is shown in FIG. 1. ESD test circuit 100 includes an ESD tester (110) and test a device (120). The ESD tester (110) includes a voltage supply (V1), a circuit ground potential (GND), a capacitor (C1), two resistors (R10 and R11), an inductor (L1), and a switch (SW1).
The voltage supply (V1) includes a ground terminal that is connected to the circuit ground potential (GND) and a power terminal that is connected to node N10. Resistor R10 is connected between nodes N10 and N11. Capacitor C1 is connected between node N11 and the circuit ground potential (GND). Resistor R11 is connected between nodes N11 and N12. Inductor L1 is connected between nodes N12 and N13. Switch SW1 is connected between nodes N13 and N14. A test control signal (TCTL) is in communication with switch SW1. Test device (120) includes a test pin (P1) that is connected to node N14 and a ground pin (P2) that is connected to the circuit ground potential (GND).
Electrostatic discharge is the direct transfer of electrostatic charge through a significant series resistor from the human body or from a charged material to the electrostatic discharge sensitive (ESDS) device. The model used to simulate this event is the Human Body Model (HBM). The Human Body Model is the oldest and most commonly used model for classifying device sensitivity to ESD. The HBM testing model represents the discharge from the fingertip of a standing individual delivered to the device. In one example, the HMB is modeled by a 100 pF capacitor(C1), a 1.5 kxcexa9 series resistor (R11), a 100 Mxcexa9 resistor (R10), and a 4 uH inductor (L1). In operation, at a first time, switch SW1 is in an open position allowing capacitor C1 to charge to the full potential of the voltage supply (V1). At a subsequent time, switch SW1 is closed by the test control signal TCTL causing the capacitor (C1) to discharge through the series combination of resistor (R11), inductor (L1), switch SW1, and into the test device (120) through test pin P1. If the test device does not have sufficient ESD protection, it will be damaged during this test.
The present invention is directed to a method and an apparatus that improves electrostatic discharge protection in a shunt regulator. An improved shunt regulator includes an xe2x80x9con-chipxe2x80x9d Miller capacitance circuit that is coupled between the drain and gate terminals of a field effect transistor (FET) shunt device. The Miller capacitance circuit provides a fast transient signal path that activates the FET to prevent damage.
Briefly stated, voltage regulators are exposed to extreme amounts of voltage over short periods of time during an electrostatic discharge (ESD) event. Shunt regulators include one or more devices that require protection from ESD events. ESD events inherently introduce extreme voltages into the shunt regulator. Capacitors are passive devices that may be used to couple high frequency signals. By providing a capacitance circuit between the gate and drain of the shunt device (or devices) in combination with a resistor circuit, a voltage can be applied to the gate of the shunt device(s) that activates the shunt device(s) in response to a fast-transient ESD event. The applied gate voltage causes the shunt device(s) to xe2x80x9cturn onxe2x80x9d, thereby providing a path for the excess voltage from the ESD event to discharge through so as to avoid catastrophic failures. A master-slave ESD protection device may be supplemented to the improved shunt regulator to further protect the shunt regulator from longer lasting ESD events.
In one aspect, the present invention is directed toward an apparatus for improving fast transient protection in a shunt circuit that includes a control terminal. The apparatus includes a protection circuit that is arranged to couple a fast transient signal to the control terminal in response to a fast transient event such that the shunt circuit is activated in response to the fast transient signal and the shunt circuit is protected from the fast transient event.
In another aspect, the present invention is directed toward an apparatus for improving electrostatic discharge protection in a shunt regulator. The apparatus includes an error amplifier circuit that is arranged to produce a control signal at a control terminal in response to a reference potential and a potential at a power supply terminal. The error amplifier has an associated amplifier response time. A capacitance circuit is arranged to couple a fast transient signal to the control terminal in response to a fast transient ESD event that occurs at the power supply terminal. A resistance circuit is arranged to produce another control signal at the control terminal in response to the fast transient signal. A shunt circuit is arranged to selectively couple power from the power supply terminal to a circuit ground terminal when activated. The shunt circuit is activated by the control signal during normal operation and the shunt circuit is activated by the other control signal during the fast transient ESD event. Excess energy from the fast transient ESD event is shunted from the power supply terminal to the circuit ground terminal by providing the other control signal to the control terminal in a time interval that is shorter than the amplifier response time.
In yet another aspect, the present invention is directed toward another apparatus for improving electrostatic discharge protection in a shunt regulator. The apparatus includes a means for amplifying that is arranged to produce a control signal at a control terminal in response to a reference potential and a regulation potential. The regulation potential is associated with a power supply terminal. A means for coupling is arranged to couple a fast transient signal to the control terminal in response to a fast transient ESD event that occurs at the power supply terminal. A means for producing is arranged to produce another control signal at the control terminal in response to the fast transient signal. A means for shunting is arranged to selectively couple power from the power supply terminal to a circuit ground terminal when activated. The shunt circuit is activated by the control signal during normal operation and the shunt circuit is activated by the other control signal during the fast transient ESD event. Excess energy from the fast transient ESD event is shunted from the power supply terminal to the circuit ground terminal by providing the other control signal to the control terminal in a time interval that is shorter than the amplifier response time.
In a further aspect, the present invention is directed toward a method of protecting a shunt device in a shunt circuit regulator from a fast ESD event on a power terminal. The method includes detecting the fast ESD event with a capacitance circuit, providing a current through the capacitance circuit in response to the fast ESD event, producing a potential in response to the current, coupling the potential to a control terminal of the shunt device such that the potential activates the shunt circuit, and coupling power from the power terminal through the shunt circuit to the circuit ground potential when the shunt device is active such that the shunt device is protected from the energy produced by the fast ESD event.
A more complete appreciation of the present invention and its improvements can be obtained by reference to the accompanying drawings, which are embodiments of the invention briefly summarized below, to the following detail description of presently preferred, and to the appended claims.